Boilers such as combination boilers are used in many homes to provide both heating and a source of instant hot water. Typically a hot water tank is dispensed with such that, when hot water is desired, cold water is drawn directly from the cold main and is passed through a heat exchanger within the boiler, the heat exchanger being directly heated by the burning of gas. The boiler only has a finite maximum power output and hence only this maximum power can be delivered into the water, less any thermal inefficiency within the boiler. Thus, the product of the flow rate of water through the boiler and the temperature rise occurring within the boiler is limited to a maximum value. In practice that means that turning the hot tap on for a fast flow might result in the delivery of warm, but not hot water and consequently to obtain hotter water the flow rate through the boiler must be reduced such that a larger temperature rise can obtained. Local climate has a significant effect on the water temperature of the cold water main. Seasonal variations within, for example, Great Britain may cause the cold water temperature to vary within a range of typically 5° C. to 20° C. Domestic boilers are generally designed to raise the cold main temperature by 35° C. at their nominal maximum flow rate. The maximum flow rate is often determined by a restrictor (often in the form of a plastic washer) within the cold water feed path to the boiler.